This invention relates to packages for integrated circuit chips; and more particularly, it relates to multichip modules which accommodate different degrees of thermal expansion among the package's components and which also enable defective chips to be easily replaced.
By an integrated circuit package or module is herein meant any airtight container which holds for one or more integrated circuit chips and which has electrical conductors that carry signals to and from the chips. Conventionally, the package is made of a ceramic substrate and a ceramic lid. This substrate and lid are shaped such that when they are placed together, a cavity is formed between them; and, one or more chips are disposed in the cavity. Solder is used to form an airtight seal between the lid and the substrate. Microscopic conductors are integrated in the substrate, and they carry electrical signals between the chips and the package's exterior.
A problem however with such prior art ceramic packages is that ceramic is a relatively poor thermal conductor in comparison to many other materials. In order to facilitate the removal of heat from the package, it is desirable to make the lid of better thermal conductor, such as a metal. However, metal has a substantially larger thermal coefficient of expansion than does ceramic; and, these expansion differences induce mechanical stress into the solder seal between the lid and the substrate when the chips heat up the package.
Further, the magnitude of the mechanical stress in the lid seal increases as the size of the package increases. Despite this, it is highly desirable to be able to make the package large so that multiple chips fit into it. Having multiple chips in one package reduces the distance and signal line capacitance between chips which improves their speed of operation, in comparison to having each chip in a separate package.
Thus, if the size of a multichip package having a ceramic substrate and metal lid is increased too much, the mechanical stress which is induced in the lid seal will become so large that it will crack the lid seal. If that occurs, contaminants can enter the package and there cause the integrated circuit chips to fail.
Also, as more chips are put into a package, the ability to be able to open and reseal the lid grows in importance. This is because the chips are expensive, so it becomes uneconomical to throw away the entire package whenever a single chip becomes defective. To enable any one defective chip to be replaced, ease of access to the chip cavity and ease of resealing the cavity lid are of major importance.
Accordingly, a primary object of the invention is to provide an improved multichip integrated circuit module which withstands large differences of thermal expansion between its lid and substrate, and which also affords ease of chip replacement.